1. Field of the Invention
The present invention relates to an active matrix liquid crystal display device, particularly of a type having improved operation speed.
2. Description of the Related Art
Conventionally, CRTs are most commonly used display devices. However, CRTs have the following problems because they use a vacuum glass tube and accelerate electrons by a high voltage:
(1) Large capacity PA1 (2) Heavy weight PA1 (3) Large power consumption. PA1 (1) The pixel pitch of the active matrix can be made smaller. PA1 (2) The reliability of the wiring connection can be improved. PA1 (3) The size of the display device can be reduced.
In view of the above, flat-panel display devices utilizing plasma or a liquid crystal are now under development.
A liquid crystal display device performs on/off display, i.e., light and shade display by controlling the polarization of light, a transmission light quantity, or a scattering light quantity by using the fact that a liquid crystal material has dielectric constants that are different in the directions parallel with and perpendicular to the molecular axis. Among generally used liquid crystal materials are a TN liquid crystal, a STN liquid crystal, and a ferroelectric liquid crystal.
Particularly in recent years, among various liquid crystal display devices, an active matrix liquid crystal display device has come to be used widely.
FIG. 7 shows an example of a conventional active matrix liquid crystal display device. In this active matrix liquid crystal display, signal lines 701 to 703 and scanning lines 704 to 706 are provided on a glass substrate in a matrix form, and thin-film transistors 707 to 710 are disposed at intersecting points of those lines. THE source electrodes of the thin-film transistors are connected to the signal lines 701 to 703, the gate electrodes are connected to the scanning lines 704 to 706, and the drain electrodes are connected to pixel electrodes (not shown) that are opposed to one of the surfaces of holding capacitors 716 to 719 and pixel region liquid crystals 712 to 715.
FIGS. 8A to 8C show voltages that are applied to the electrodes of a thin-film transistor. As shown in FIG. 8A, an electric signal V.sub.S is applied to the source electrode of the thin-film transistor via signal lines. As shown in FIG. 8B, an electric signal V.sub.G is applied to the gate electrode of the thin-film transistor via scanning lines. In accordance with the signals V.sub.S and V.sub.G, a voltage V.sub.D of the drain electrode has a waveform shown in FIG. 8C.
Where the thin-film transistor is of an N-channel type, when the gate voltage becomes high (positive), the thin-film transistor is turned on to equalize the source voltage and the drain voltage. As a result, the voltage of the signal line is written to the holding capacitor. When the gate voltage becomes low (negative), the thin-film transistor is turned off to electrically separate the source and drain electrodes. As a result, the voltage of the holding capacitor is held until the thin-film transistor is turned on next time to cause writing.
The liquid crystal element (indicated by 712 to 715 in FIG. 7) that is interposed between the opposed electrode and the pixel electrode receives a difference of voltages of those electrodes, and its light polarizing characteristic is varied in accordance with the difference voltage. By inserting a polarizing plate, light and shade display is obtained in accordance with the light polarizing state of the liquid crystal element.
Conventional active matrix liquid crystal display devices employ a TN liquid crystal. With a polarizing plate inserted, a TN liquid crystal exhibits a transmittance-applied voltage (V) characteristic as shown in FIG. 10. Having a relatively gentle slope, this transmittance-applied voltage (V) characteristic enables gradational display as controlled by the applied voltage.
However, because TN liquid crystals generally do an effective-value response, they have a problem of slow response to an applied voltage.
In a TN liquid crystal, usually, when the gradation level changes from black to white (see FIG. 11) or vise versa, there occurs a response delay of 10 msec to several tens of milliseconds. That is, the liquid crystal cannot respond until lapse of 10 msec to several tens of milliseconds after the voltage application.
In conventional active matrix liquid crystal display devices, in displaying a certain gradation level, a voltage applied to a liquid crystal display device is considered constant with a lapse of time; that is, the response of a liquid crystal is not taken into consideration.
Therefore, although conventional active matrix liquid crystal display devices can exhibit display performance which is equivalent or superior to that of CRTs in displaying a still picture, they cannot provide image quality equivalent to that of CRTs in displaying a moving picture due to the above-described delayed response.
Conventionally, there are two kinds of methods of producing pixel drive circuits. According to the first method, they are produced as transistor integrated circuits of single crystal silicon. According to the second method, they are produced as thin-film transistors using polysilicon so as to be formed on the same substrate as an active matrix in an integral manner. In the first method, it is a general procedure that the drive circuits are externally provided and connected to an active matrix substrate in the form of TAB or COG. In the second method, the drive circuits are formed on the same substrate as the active matrix and connected thereto by metal wiring. FIG. 9 shows an example of a liquid crystal display device that incorporates drive circuits constituted of polysilicon thin-film transistors.
Therefore, the second method is advantageous over the first method in the following points:
Where the active matrix is driven by use of TAB, the pitch of the active matrix cannot be made smaller than a certain value because the TAB pitch cannot be made smaller than a value that allows bonding to the glass substrate. In the second method, in which the drive circuits are incorporated in the substrate and therefore there exists no bonding to the active matrix, the matrix pitch can be reduced without any TAB-related limitation.
In the case of using TAB, several thousands wires come out from the active matrix. Therefore, wire breaking occurs at a high probability at connection points between TAB and the active matrix substrate. On the other hand, where the drive circuits are incorporated in the active matrix substrate, the number of terminals of the substrate for external connection is about 1/100 of the number in the case of using TAB. Thus, an improvement in the reliability is expected.
Where TAB is employed in a display device, such as a view finder, having a small screen, TAB of the drive circuits is larger than the active matrix, resulting in a limitation in reducing the capacity of a video camera and the like. On the other hand, where the drive circuits are incorporated in the substrate, the circuit width can be made smaller than 5 mm, contributing to the size reduction of such display devices as a view finder.